The big picture: Variation in animal reproduction


  • Mating between males and females is boring
    • species can reproduce without sex
    • individual of species can change sex
    • species have male and female sex organs
    • some populations have only a few that mate


  • Two modes of reproduction in animal kingdom
    1. Sexual:
    2. Asexual:


  • Animals exhibit cycles of reproductive activity


  • Mammalian reproduction is governed by hormones

Asexual reproduction in the animal kingdom


  • Budding:
    • found among invertebrates


  • Fission:
    • about the same size
    • found among invertebrates


  • Fragmentation:
    • accompanied by regeneration of lost body parts
    • sponges, worms, starfish


  • Parthenogenesis:
    • found among invertebrates and vertebrates

Review video on budding, fragmentation (+ unnecessary extras)

Parthenogenesis: Male drone bees are fertile and haploid!


Animal sex blurs the lines of male and female



  • Sexual reproduction commonly between male and female
    • finding a partner is difficult


  • Stationary animals tend to be hermaphroditic
    • individual have male and female reproductive bits
    • Any 2 individuals can mate/self-fertilize


  • Sex reversal occurs in different animal phyla
    • oysters are males first, then turn to female when larger
    • the largest female in some coral fish turn to male

Reproductive cycles are common in the animal kingdom



  • Cycles controlled by hormones and environmental cues
    • occur with asexual or sexual modes
    • usually related to seasons


  • Ovulation: release of eggs at midpoint of female cycle
    • cycles are variable in length, synced with seasons
    • sheep: cycle + pregnancy = birth in spring


  • Daphnia: cycles between asexual and sexual
    • seasonally related to environment

The “Handicap” of sexual reproduction


How sperm and egg come together…





  • Fertilization =
    • can be external or internal



  • External: eggs shed by the female & fertilized by sperm in the environment
    • moist habitat always required
    • spawning:

How sperm and egg come together…





  • Fertilization:
    • can be external or internal



  • Pheromones: chemicals used to influence physiology and behavior
    • function as mate attractants
    • pheromone production in humans is controversial

Pheromones in humans?


Internal vs External: Offspring survival





  • Internal fertilization produces fewer gametes
    • survival of zygotes higher


  • Internal associated with protection mechanisms
    • parental care of young
    • shells, amniotic eggs, marsupials(?)


  • Not just vertebrates..

Making and delivering gametes



  • Individuals usually have gonads that produce gametes
    • essential for sexual reproduction
    • often established early in embryo development


  • Many animals have complex systems
    • structures to carry, store, protect and nourish gametes
    • e.g. fruit fly (spermathecae stores sperm in female)


  • Mating may occur with more than 1 male
    • males have mechanisms to prevent other male success

Mammalian reproduction governed by hormones




  • Hormones coordinated from many glands/organs
    • hypothalamus, pituitary and gonads
    • endocrine system


  • Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus
    • directs release of FSH (follicle-stimulating hormone)
    • directs release of LH (luteinizing hormone)
    • both from pituitary gland


  • FSH and LH act on male and female gonads
    • stimulate sex hormone production

Mammalian reproduction governed by hormones



  • Gonads then produce and secrete 3 hormones
    • testosterone, estradiol and progesterone
    • found in male/females in different amounts


  • Male embryos: direct reproductive structures


  • Maturing mammals: induce sex characteristics
    • physical/behavior differences (male/female)


  • Mature mammals: play role in gamete formation

Hormonal control of male system (negative feedbacks)


Most female mammals have an ovarian cycle



  • Some females have linked menstrual cycle
    • 10 primates, elephant shrew, some bats, spiny mouse


  • Ovarian cycle:
    • series of changes in ovaries


  • Menstrual cycle:
    • endometrium thickens with blood vessels
    • preparation for embryo implantation
    • if embryo does not implant, endometrium is shed


  • Both cycles occur concurrently
    • use same series of hormones